Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/777939
32 AGGREGATES MANAGER / February 2017 SPECIAL REPORT be designed to minimize dust loading on the intake filtering unit by using one of two proven techniques: (A) a self-cleaning filter technique, or (B) a centrifugal design which spins out the over-sized dust particles (>5.0 microns) before reaching the intake filter. A common self-cleaning method is to use a re- verse-pulse or back-flushing technique in which a compressed air system is activated to blow the dust cake off the intake filter. This reverse-pulse can be used either on a regular time interval or based upon a differential pressure across the filter. With the centrifugal design, the system spins the oversized particles out of the unit and back into the atmosphere to minimize the num- ber of particles being deposited on the intake filter. This system has an approximately 90 percent efficiency with particles great- er than 5 microns. Both the self-cleaning and centrifugal techniques have been tested and shown to be very effective at providing a known quantity of intake air to the enclosed cab while minimiz- ing dust loading on the intake filter. 2. Use of an in-cab pressure monitor to determine the integrity and functioning of filtration and pres- surization systems. With any new filtration and pressurization system, the starting positive pressure should be established and recorded. Then, over time, as the system filters load with dust and contaminants, the intake airflow decreases, which also decreases the cab's positive pressure. Since filter loading rates are different in all cases based on contaminant levels where the equipment is located in the mine, using a filter cleaning or changing schedule based on time is not the preferred method, because a mechanical filter becomes more efficient as it loads with dust and contaminants and devel- ops what is commonly known as a "dust cake." Knowing the positive pressure of the cab provides the equipment operator or maintenance personnel with the ideal filter changing time based upon the required intake airflow. The enclosed cab's positive pressure will be highest with new filters and then as the filters load with dust, the intake airflow will decrease, which also directly decreases the cab's positive pressure. Once the cab pressure decreases to the point where the intake air is at the minimal level, it signifies that a new intake filter needs to be installed. Conversely, a rapid increase in positive cab pressure also indicates a system failure. This could include such things as a hole or tear in the filter media, a clog in the recirculation system such as a plastic bag or a rag covering the recirculation inlet, or even a maintenance worker removing a used filter and then forgetting to replace it with a new one. It is also possible, over time, for the cab integrity to be compromised by a damaged door, window gasket, or seal being damaged or removed. An in-cab pressure monitor provides the most real-time in- dication of the cab's performance over time. There are a number of commercially available pressure gauges that have an alarm option to signify that the intake filter needs to be changed. Using one of these pressure gauges is a very effective way to monitor the cab filtration and pressurization system functionality. 3. Changes in air quality using different filter con- figurations and filter dust loading characteristics in the filtration and pressurization systems. Different filter combinations were evaluated in both pieces of equipment during the testing performed at the Shelly Materi- als Co. mine. Table 1 shows the various combinations and the resulting protection factors on the roof bolter machine. These tests were performed under static conditions which indicates that no one was entering or exiting the enclosed cab during the particle count testing, and thus no dust was allowed to enter by opening the cab door, nor was anyone inside the enclosed cab to stir up any in-cab dust sources. Because of this, these protection factor (PF) values are at their highest. Table 1: Roof bolter protection factors with various filter combinations. FILTER COMBINATIONS Used intake and recirculation filters 3 PROTECTION FACTORS Used intake, recirculation, and final filters 76 PROTECTION FACTORS New intake and final filters 300 PROTECTION FACTORS Used intake and final filters 465 PROTECTION FACTORS These results provide a significant amount of insight into the effectiveness of the various filters in the system. Obviously, the lowest PF measured was when testing a used intake and recirculation filter combination with no final filter. In the next evaluation, a final filter was added to the system, resulting in a significant increase in the protection factor from a value of 3 to 76. Somewhat surprising was the huge increase in the next two combinations when the recirculation filter was removed from the system. A PF of 300 was achieved with a new intake and final filter, which was a substantial increase over the previous values. The researchers do not believe this would be typical with most filtration and pressurization systems, but for this system, adding the recirculation filter into the system was detrimental because the recirculation filter was under-sized, which significantly reduced the recirculation airflow and thus, the air cleaning potential of the system. A negative aspect of not having the recirculation filter in the system is that dirt and dust from inside the cab is drawn into and is deposited in the HVAC system, thereby increasing maintenance issues. An alternative solution to improving this cab filtration system would be to increase the size of the recirculation filter to increase its airflow capabilities. Finally, note the significant increase in the protection factor from 300 to 465 when evaluating a used intake and final filter. As stated previously, a mechanical filter becomes more efficient as it loads with dusts and contaminants and creates a dust cake, which results in a significant increase in the air quality and the resulting PF.